The spine is a flexible column formed of a plurality of bones called vertebrae. The vertebrae are hollow and piled one upon the other, forming a strong hollow column for support of the cranium and trunk. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies. Various spinal disorders may cause the spine to become misaligned, curved, and/or twisted or result in fractured and/or compressed vertebrae. It is often necessary to surgically correct these spinal disorders.
Spinal fixation systems may be used in surgery to align, adjust, and/or fix portions of the spinal column, i.e., vertebrae, in a desired spatial relationship relative to each other. Many spinal fixation systems employ spinal rods for supporting the spine and for properly positioning components of the spine for various treatment purposes. Vertebral anchors, comprising pins, bolts, screws, and hooks, engage the vertebrae and connect the supporting rod to different vertebrae. The size, length, and shape of the cylindrical rod depend on the size, number, and position of the vertebrae to be held in a desired spatial relationship relative to each other by the apparatus.
During spinal surgery, a surgeon first exposes the spine posterior and attaches the vertebral anchors to selected vertebrae of the spine. The surgeon then inserts a properly shaped spinal rod into rod-receiving portions of the vertebral anchors to connect the selected vertebrae, thereby fixing the relative positions of the vertebrae. Generally, a controlled mechanical force is required to bring together the spinal rod and a spinal implant, such as the vertebral anchors, in a convenient manner. After insertion, a surgeon must insert a locking mechanism, such as a set screw, into the vertebral anchor to lock the spinal rod to the implant after the force for inserting the rod is removed.
The spine is formed in motion segments with each segment represented by two vertebrae and the structures that connect them. The segments allows for six degrees of freedom of movement, resulting in six components of motion for each vertebra V. The movement may be characterized as translation and rotation on each of three axes (X, Y, and Z) forming a Cartesian coordinate system of each vertebra as illustrated in FIG. 1. Each axis is perpendicular to a plane. X is perpendicular to the coronal plane and may be referred to as the coronal axis, Y is perpendicular to the sagittal plane and may be referred to as the sagittal axis, and Z is perpendicular to the transverse plane and may be referred to as the transverse axis.
The axes of each vertebra in a normal standing, static spine, exhibit no rotation in the coronal or transverse planes and a gentle S-shaped curvature in the sagittal plane. In the coronal plane, the vertebrae are normally aligned and present neutral rotation. In the transverse plane, the vertebrae are likewise normally aligned and present neutral rotation. Therefore, the X and Z axes of the different vertebrae are substantially coplanar. In the sagittal plane, the vertebrae present a certain degree of rotation and translation which form the physiological S-shaped curvature: namely, cervical lordosis, thoracic kyphosis, and lumbar lordosis. Therefore, the coronal (X) axes of the thoracic vertebrae are posteriorly divergent in kyphotic segments while they are posteriorly convergent in lordotic segments.
Spinal deformities of varying etiologies which alter the natural alignment of the spine are well known. Such deformities include abnormal spinal curvatures such as scoliosis, kyphosis, and/or other abnormal curvatures. With specific regard to scoliotic deformities, the abnormal curvature of the spinal column is three-dimensional as illustrated in FIG. 2. Specifically, scoliotic deformities can be separated into abnormal translation and/or rotation of the vertebrae in each of the coronal, transverse, and sagittal planes. For example, in a deformed spine, the vertebrae may be rotated and translated in all three planes, as illustrated by arrows R and T, resulting in loss of the normal coplanar alignment of the Y and Z axes. A hypokyphosis may also be present in the thoracic region of the spine causing loss of the normal posterior divergence of X axes. Therefore, treatment of spinal deformities should preferably be aimed at addressing reduction of the abnormal curvature in each of the three spatial planes.
Surgical correction of the rotation and translational alignment of one or more vertebrae in the spinal column typically requires repositioning and re-alignment of the various motion segments. Individual correction of each segment can be time-consuming, cumbersome, and potentially difficult to achieve during a surgical procedure. For example, the alignment of multiple vertebral levels can require manipulation of instrumentation at each level to achieve the desired results. Forces applied to the vertebral body need to be controlled to minimize stresses on the vertebrae and associated implants. Furthermore, alignment at one level often must be maintained while other levels are aligned. Often the instrumentation employed to achieve the alignment can hinder placement of stabilization constructs, such as fixation rods, that post-operatively maintain the corrected positioning and alignment achieved during surgery.
Various individual instruments associated with existing systems and methods may perform individual tasks associated with the following operations: segmental vertebral body alignment, en bloc simultaneous derotation of multiple levels of vertebral bodies, reduction of a fixation rod to an implant head, and stabilization of corrected alignment while setscrews are tightened. These instruments, systems, and methods may facilitate surgical correction of the alignment and positioning of a vertebra or vertebrae of the spinal column, placement of stabilization constructs that post-operatively maintain the corrected vertebra or vertebrae, and facilitate control of the stress exerted on implants and vertebrae to which the implants are attached. However, none of these instruments, systems, and methods is capable of performing all four tasks.